1. Field of the Invention
The present invention relates to a layer forming apparatus for forming a deposition layer on a substrate.
2. Description of the Prior Art
As an apparatus for forming a deposition layer on a substrate, for example for the purpose of efficiently producing an electrophotographic photosensitive drum, there is already known a structure as will be explained in the following in relation to FIG. 1.
A cylindrical sealed box 1 is provided therein with a coaxial cathode electrode 3 supported by means of annular insulators 2A, 2B, and is equipped at the upper end with a gate valve 4. Inside said cathode electrode 3 there are provided plural vertical gas emission pipes 5 for a gaseous reagent, of which lower ends penetrate the lower end of the sealed box 1 and are connected to a gas introducing pipe 7. Each of said gas emission pipes 5 is provided, in the sealed box 1, with plural gas emission holes 5A facing the cathode 3, so that a gaseous reagent from an gas source (not shown) is supplied through the gas introducing pipe 7 to the gas emission pipes 5 and is emitted inside the cathode 3 through said gas emission holes 5A. In the bottom of the sealed box 1, there open plural gas exhaust pipes 6, of which the other ends are connected to a gas exhaust main pipe 8 in turn connected to gas discharge means (not shown), so that the gas in the sealed box 1 is discharged by said discharge means through said gas exhaust pipes 6 and said gas exhaust main pipe 8.
The cathode 3 is provided, on the external periphery thereof, with a spiral groove 3A, which is connected to a supply pipe 9 and a discharge pipe 10 for cooling water respectively at the lower and upper ends. The cathode 3 is therefore cooled by supplying cooling water from the supply pipe 9 into the groove 3A and discharging it from the discharge pipe 10.
A cylindrical substrate 11 for the electrophotographic photosensitive drum is supported inside the cathode coaxially with the same, by engaging a support plate 16, formed in the upper part of said substrate, with a head 12A at the upper end of a rod 12 positioned on the central axis of the cathode 3. The cylindrical substrate 11 is grounded through the rod 12, while the cathode 3 is connected to an RF power source 17. Outside the rod 12, and inside the cylindrical substrate 11, there is provided a heater 15 fixed to the bottom of the sealed box 1. The lower end of the rod 12 is linked, under the lower end of the sealed box 1, to a motor 13 through a reducing mechanism 14, whereby the cylindrical substrate 11 is rotated by said motor 13. Though not illustrated, the interior of the sealed box 1 is evacuated by gas discharge means.
The above-described structure can deposit, for example, a layer of amorphous silicon on the external periphery of the cylindrical substrate 11, by at first evacuating the interior of the sealed box 1, then emitting a gaseous reagent into the sealed box 1 through the gas emission pipes 5, turning on the heater, rotating the cylindrical substrate 11 and supplying a high-frequency electric power from the RF power source 17 to the cathode 3 thereby causing a discharge between the cathode and the cylindrical substrate 11.
The above-described conventional layer forming apparatus is, however, associated with the following drawbacks.
Decomposition of gaseous reagent causes deposition of a polysilane layer also on the internal periphery of the cathode 3 and the sealed box 1, and such polysilane layer has to be removed after each layer formation, by means for example of dry etch cleaning. However such dry etch cleaning requires a long time, for example about 2 hours, thus significantly deteriorating the production efficiency of the electrophotographic photosensitive drums. Said dry etch cleaning for example employs a gaseous mixture of CF.sub.4 and oxygen, which are decomposed in high-frequency plasma to gasify the polysilane film deposited on the internal periphery of the cathode etc. Thus, in order to withstand strongly corrosive gasses generated in the dry etch cleaning process, the cathode, gas emission tubes etc. have to be made of anticorrosive materials, use of which inevitably increases the cost of the apparatus. In addition, the cost of the gasses to be employed in the cleaning process has to be considered. These factors have lead to a high cost of layer formation.